Japanese schools run on a three term system, as opposed to two terms most other countries use, and the Japanese school year starts after spring break in april. If she goes to France after spring break she will only catch the tail end of the French school year instead of the beginning.
https://web-japan.org/kidsweb/explore/calendar/april/schoolyear.html
https://www.japantimes.co.jp/opinion/2020/06/16/commentary/japan-commentary/no-end-sight-debate-japans-school-year/#.X0YMp0GexaQ